Sediments of various lithologies cored from most parts of the deep ocean floor have been found to contain a record of the past behaviour of the Earth's magnetic field, especially the sequence of reversals over at least the past 5 m.y. (ref. 1). The mechanism by which these sediments acquired their natural remanent magnetism (NRM) is, however, still poorly understood. We have recently conducted experiments that indicate that post-depositional remanent magnetisation (ref. 2) is a viable mechanism of magnetisation of deep-sea sediments.